Meet Hadvar and his men on the perimeter of the fortress. Their reconaissance reveals a grate, whose existence has eluded the Stormcloaks, that leads you straight into the fort undetected. Head to the grate and infiltrate the fort.

With or without Sneakingskills, proceed through the fort, busting up the Stormcloak rabble. Each soldier should have a Fort Kastav Prison Key on his body. This should let you easily unlock the prison doors incarcerating fellow Imperial men.

Once freed, take them outside to the courtyard as instructed and battle the big cluster of Stormcloaks outside. The freed Imperial plus Hadvar's men will help you. Speak to Hadvar to complete the quest and report to General Tullius for the next step.